1. Field of the Invention
The present invention relates to a process for the batchwise purification of crude acrylic acid, which comprises at least one stripping stage and at least one purification stage.
2. Description of the Background
Acrylic acid is a key chemical which is used, for example, as such or in the form of its alkyl esters for the preparation of polymers produced by free radical polymerization.
Usually, acrylic acid is prepared by gas-phase oxidation of propene and/or propane under heterogeneous catalysis (cf. for example DE-A 19 508 558, EP-A 293 224, EP-A 257 565 and the literature cited in these publications).
The gas mixture leaving the gas-phase oxidation does not, of course, consist of pure acrylic acid but of a gas mixture which contains the latter and from which acrylic acid has to be isolated.
The various known methods for isolating acrylic acid from the product gas mixture of the gas-phase oxidation are summarized in, for example, DE-A 19 600 955.
A common feature of the known separation methods is that the desired acrylic acid is first isolated from the reaction gas mixture either by absorption using a solvent (cf. also DE-A 4 308 087) or by partial condensation. The resulting absorbate or condensate is then worked up, as a rule by distillation (with or without the addition of an azeotropic entraining agent) and/or extraction, to give an acrylic acid quality which usually contains, based on its weight, ≧95% by weight, frequently ≧96% by weight or ≧97% by weight, often ≧98% by weight, in some cases ≧99% by weight and in some cases even ≧99.5% by weight of acrylic acid.
Acrylic acids of the abovementioned qualities are all to be referred to in this publication very generally by the term crude acrylic acid. Typically, they contain at least one of the following impurities: water, acetic acid, propionic acid, low molecular weight aldehydes, such as acrolein, furfural or benzaldehyde, esters of acrylic acid and allyl alcohol, maleic anhydride, a process polymerization inhibitor, e.g. phenothiazine and/or N-oxyl radicals and diacrylic acid (the adduct formed by
Michael addition of acrylic acid with itself when mixtures containing acrylic acid are left to stand).
When acrylic acid is used, for example for free radical polymerization purposes, the majority of the abovementioned impurities prove to be troublesome (for example, they produce discolorations of the resulting polymer or influence the induction time, i.e. the time until the beginning of the polymerization, in a disadvantageous manner).
A particularly undesirable impurity proves to be diacrylic acid. Although it generally undergoes free radical copolymerization, it does so with the disadvantage that it cleaves when the polymer is subjected to a thermal load and liberates monomeric acrylic acid, which cannot be tolerated particularly when the polymer is used in the hygiene sector (keyword: superabsorbers in babies' diapers; these polymers are partially neutralized polyacrylic acids obtained by free radical polymerization).
Usually, crude acrylic acid is therefore not used as such but only after further purification to give pure acrylic acid.
The literature (cf. e.g. EP-A 616 998) proposes carrying out the purification of crude acrylic acid to pure acrylic acid by batchwise crystallization. There, the initial crude acrylic acid is separated by the action of low temperatures, in a first crystallization step, into acrylic acid crystals having a higher purity than the initial crude acrylic acid and into a mother liquor having a lower purity than the initial crude acrylic acid. The crystallization can be carried out both statically (the melt is at rest during the crystallization, e.g. plate-type crystallizer or ribbed-tube crystallizer) and dynamically (the melt is agitated during the crystallization, e.g. falling-film crystallizer or tube with flow over the whole cross section). The resulting acrylic acid crystals are melted and, if required, further purified in one or more further crystallization steps carried out in succession.
To ensure that the yield is economical, the mother liquor obtained in the first crystallization step is also subjected to at least one further crystallization step. The crystallization steps in which the melt to be crystallized originates from the acrylic acid crystals obtained in the first crystallization step are generally referred to as purification stages. Likewise, the first crystallization step is referred to as the purification stage. In contrast, all crystallization steps in which the melt to be crystallized has originated from the mother liquor obtained in the first crystallization step are referred to as stripping stages.
This means that a batchwise purification of crude acrylic acid by crystallization usually comprises at least one purification stage and at least one stripping stage.
For the batchwise purification of crude acrylic acid, too, the prior art (e.g. EP-A 616 998) recommends carrying out both the one or more purification stages (i.e. including any additionally used purification stages) and at least the first stripping stage in one and the same crystallizer, i.e. in a single crystallizer. The disadvantage of this procedure is that those fractions of the initial crude acrylic acid which are not subjected to a crystallization in the crystallizer are temporarily stored in containers. During this temporary storage, however, diacrylic acid forms. In a subsequent crystallization stage, the diacrylic acid formed during the temporary storage is separated off, but the diacrylic acid separated off is equivalent to a lower yield of pure acrylic acid.